It is known that an asynchronous motor can be supplied from a static frequency convertor, for example for achieving a high speed of the motor or for making possible control of the speed of the motor. However, in frequency convertor operation additional losses harmonic losses, will arise because of harmonics in the motor current. By designing the motor so that its leakage reactance (short-circuit reactance) is high, a reduction of the harmonic currents, and hence of the additional losses, is obtained. However, an increase of the leakage reactance entails a reduction of the maximum torque of the motor and of its power factor, and it is therefore possible only to a limited extent to reduce the additional losses in this way.
From Swedish published patent application 433 995 it is known that the additional losses in an asynchronous motor supplied by a frequency convertor may be reduced by providing each winding bar with an axially extending channel facing the slot opening. However, this solution has several drawbacks. It is not possible to use it in rotor windings of a cast design. It gives a relatively low value of the differential leakage reactance of the motor and hence still relatively high harmonic currents and additional losses. In the solution mentioned it is not possible to effectively prevent the field in the vicinity of the slot opening from penetrating into the rotor conductor and generating additional losses there. Further, in this known solution it is the eddy currents in the rotor conductor that reduce the field penetrating into the conductor, and the effect achieved is therefore inseparably connected with a certain degree of additional losses.